ESA has tested a air breating system for low altitude satellites that could keep them in orbit almost indefinitely. You could keep a satellite in a low orbit (say 200 km) with a ion engine (like ESA did the GOCE mission in 2009-2013), but of course one day you run out of (xenon) fuel. ESA now has developed a Ram-Electric Propulsion system that can breathe the air in the upper atmosphere. They replaced the xenon fuel with a mixture of nitrogen and oxygen that you would get in the upper atmosphere. The engine plume has a different colour with this mixture: purple instead of blue. And yes, it does produce enough thrust to overcome the drag (... as long as your satellite is aerodynamic enough I suppose).